THE COLLOIDAL PROPERTIES OF SOIL 



151 



complete precipitation, since calcium phosphate is insoluble. 

 The precipitated potassium compound dissolves somewhat in 

 water, but it has no definite solution pressure, instead the 

 amount of potassium dissolving increases with the amount 

 present. It can also be decomposed by sodium salts ; hence 

 addition of sodium sulphate to the soil increases the amount 

 of soluble potassium ions and to this extent acts like a dress- 

 ing of potassic manure. Magnesium salts have a similar 

 effect, and, like sodium salts, lead to an increase in the amount 

 of potassium available for the crop. Some of Lawes and 

 Gilbert's results (1660) are given in Table XLI. 



TABLE XLI. EFFECT OF SODIUM AND MAGNESIUM SULPHATES IN INCREASING 

 THE SUPPLY OF POTASH TO THE PLANT. LAWES AND GILBERT (i66a). 



In the twenty years the sodium sulphate has enabled the 

 plant to take up an additional 263 lb. of K 2 O, whilst the 



